The present invention relates generally to the field of waveguides and more specifically to the field of waveguides suitable for providing low loss versatile bending of electromagnetic energy in the TE.sub.01 circular waveguide mode without generating unwanted parasitic modes. Theory derived for ideal lossless lines shows that when a TE.sub.01 electromagnetic wave enters a bend, energy is transferred from the TE.sub.01 mode to the TM.sub.11 mode. The transfer of energy continues until the critical angle is reached where all of the TE.sub.01 mode has been converted to the TM.sub.11 mode. Beyond this critical angle, energy is transferred back from the TM.sub.11 mode to the TE.sub.01 mode and when a total angle of twice the critical angle is reached, all the energy, in the ideal case, is transferred back into the TE.sub.01 mode. Since the critical angle is determined by the free space wavelength and the diameter of the waveguide, the critical bend angle is frequency sensitive but bandwidths of about 10% are readily achieved.
The energy transfers in this manner because the TE.sub.01 and the TM.sub.11 modes are degenerate, i.e., they have the same phase velocity. Therefore, the energy transfer between modes is a continuous process; the waves add coherently, regardless of where the transfer occurs and the transfer is theoretically independent of the bend radius. Because of this degeneracy, a gradual large radius bend is no better than a more abrupt short radius bend.
If this mode degeneracy is removed by modifying the waveguides so that the phase velocity of the two modes is no longer the same, the transfer of energy between the modes is reduced and gradual large radius bends are better than short radius bends.
Older methods used to remove this degeneracy have utilized threaded copper pipes and dielectric inserts to guide the TE.sub.01 circular waveguides modes through a bend. These devices, however, are difficult to fabricate out of solid material and are complicated in structure. In addition, the final configuration must be determined at the onset of fabrication. A symmetrical corrugated structure has also been fabricated by crimping brass or beryllium copper to form a flexible section. The disadvantage with this symmetrical configuration is that the symmetrical corrugation disturbs the integrity of the inside conducting wall of the waveguide more than is necessary, thus increasing losses. Also, the use of brass or beryllium copper is lossier by nature than is pure copper. Additionally, it is also difficult to maintain the precise period of the corrugation over a specified length using mechanical crimping methods.